The present invention relates to thin-film transistors (TFTS) and also to a method of fabricating TFTs. Furthermore, the invention relates to a semiconductor circuit using plural TFTs and also to a method of fabricating such a semiconductor circuit. A thin-film transistor fabricated according to the present invention is formed either on an insulating substrate of glass or the like or on a semiconductor substrate of a single crystal of silicon. Especially, where the invention is applied to a semiconductor circuit comprising a low-speed matrix circuit and a high-speed peripheral circuit for driving the matrix circuit such as a monolithic active-matrix circuit used in a liquid-crystal display or the like, great advantages can be obtained.
In recent years, an insulated gate semiconductor device having an active layer (also called an active region) in the form of a thin film has been investigated. Especially, an insulated-gate transistor in the form of a thin film which is known as a TFT has been earnestly investigated. Transistors of this kind are formed on a transparent insulating substrate and used either to control each pixel in a display device such as a liquid-crystal display having a matrix structure or to form a driver circuit. Depending on the material or the state of crystallization of the used semiconductor, they are classified as amorphous silicon TFTs or crystalline silicon TFTs.
Generally, amorphous semiconductors have small field mobilities and so they cannot be used in TFTs which are required to operate at high speeds. Accordingly, in recent years, crystalline silicon TFTs have been investigated and developed to fabricate circuits of higher performance.
Since crystalline semiconductors have higher field mobilities than amorphous semiconductors, the crystalline semiconductors are capable of operating at higher speeds. With respect to crystalline silicon, PMOS TFTs can be fabricated, as well as NMOS TFTs. For example, it is known that the peripheral circuit of an active-matrix liquid-crystal display is composed of CMOS crystalline TFTs similarly to the active-matrix circuit portion. That is, this has a monolithic structure.
FIG. 3 is a block diagram of a monolithic active-matrix circuit used in a liquid-crystal display. A column decoder 1 and a row decoder 2 are formed on a substrate 7 to form a peripheral driver circuit. Pixel circuits 4 each consisting of a transistor and a capacitor are formed in a matrix region 3. The matrix it region is connected with the peripheral circuit by conductive interconnects 5 and 6. TFTs used in the peripheral circuit are required to operate at high speeds, while TFTs used in the pixel circuits are required to have a low-leakage current. These are conflicting characteristics in terms of physics but it is necessary that these two kinds of TFTs be formed on the same substrate at the same time.
However, all TFTs fabricated by the same process show similar characteristics. For example, TFTs using crystalline silicon fabricated by thermal annealing, TFTs used in the matrix region, and TFTs in the peripheral driver circuit all have similar characteristics. It has been difficult to obtain a low-leakage current suited for the pixel circuits and a high mobility adapted for the peripheral driver circuit at the same time. It has been possible to solve the above difficulty by using thermal annealing and crystallization using selective laser annealing at the same time. In this case, TFTs fabricated by thermal annealing can be used in the matrix region, whereas TFTs fabricated by laser annealing can be employed in the peripheral driver circuit region. However, the crystallinity of silicon crystallized by laser annealing has quite low homogeneity. Especially, it is difficult to use these TFTs in a peripheral driver circuit which is required to be defect-free.
It is also possible to use crystallization relying on laser annealing in order to obtain crystalline silicon. If a semiconductor circuit is fabricated from this silicon crystallized by laser annealing, TFTs in the matrix region and TFTs in the peripheral driver circuit all have similar characteristics. Accordingly, an alternative method of crystallizing silicon may be contemplated. In particular, TFTs in the matrix region are formed, using thermal annealing. TFTs in the peripheral driver circuit are formed, using laser annealing. However, where the thermal annealing is adopted, the silicon must be annealed at 600xc2x0 C. for as long as 24 hours, or the silicon must be annealed at a high temperature exceeding 1000xc2x0 C. In the former method, the throughput drops. In the latter method, the material of the usable substrate is limited to quartz.
It is an object of the present invention to provide a method of fabricating a semiconductor circuit without relying on a complex process or without deteriorating the production yield or the cost.
It is another object of the invention to provide a method of easily mass-producing two kinds of TFTs with minimum variations in the process, one of the two kinds being required to have a high mobility, the other being required to have a low-leakage current.
Our research has revealed that addition of a trace amount of a catalytic material to a substantially amorphous silicon film promotes crystallization, lowers the crystallization temperature, and shortens the crystallization time. Examples of the catalytic material include simple substances of nickel (Ni), iron (Fe), cobalt (Co), and platinum, and silicides thereof. More specifically, a film containing such a catalytic element, particles of the element, or clusters of the element are used to form a first film on or under an amorphous silicon film such that the first film is in intimate contact with the amorphous silicon film. Alternatively, such a catalytic element is implanted into an amorphous silicon film by ion implantation or other method. Then, the film is thermally annealed at an appropriate temperature, typically below 580xc2x0 C., in a short time within 8 hours. As a result, the amorphous film is crystallized.
Where a film is fabricated from such a catalytic element, the concentration of the element is sufficiently low and so the film is quite thin. To form this film, a method using a vacuum pump such as sputtering or vacuum evaporation can be employed. In addition, a method which can be effected under atmospheric pressure such as spin coating or dipping can be utilized. This atmospheric-pressure method is easy to perform and provides high productivity. In this case, an acetate, a nitrate, an organic salt, or the like containing such a catalytic element is dissolved in an appropriate solvent, and the concentration is adjusted to an adequate value.
When the amorphous silicon film is formed by CVD, the catalytic material is added to the raw material gases. When the amorphous silicon film is formed by physical vapor deposition such as sputtering, the catalytic material may be added to the target or evaporation source for forming a film. Of course, as the anneal temperature rises, the crystallization time decreases. Furthermore, as the concentrations of nickel, iron, cobalt, and platinum are increased, the crystallization temperature drops, and the crystallization time is shortened. Our research has revealed that it is necessary that the concentration of at least one of these elements be in excess of 1017 cmxe2x88x923, in order to promote crystallization. Preferably, the concentration is in excess of 5xc3x971018 cmxe2x88x923.
Since all of the aforementioned catalytic materials are not desirable for silicon, it is desired that their concentrations be made as low as possible. Our research has shown that the total concentration of these catalytic materials is preferably not in excess of 1xc3x971020 cmxe2x88x923. Also, local concentrations (e.g., those at grain boundaries) are preferably not in excess of 1xc3x971020 cmxe2x88x923.
In the present invention, TFTs which operate at high speeds and are used as TFTs for driving an active-matrix circuit are selectively formed by laser crystallization. On the other hand, TFTs which operate at relatively low speeds and are used as low-leakage current TFTs of pixels of the active-matrix circuit are fabricated by making positive use of the features of crystallization promoted by the catalytic elements described above. To form the latter TFTs, silicon is crystallized at a low temperature in a short time. As a result, a circuit comprising transistors that achieve both low-leakage current and high-speed operation which would normally be conflicting characteristics can be formed on the same substrate.
We also discovered that when a film containing catalytic elements such as nickel, iron, cobalt, platinum, and palladium is irradiated with laser light or other equivalent intense light, a quite rapid crystal growth takes place even if the concentration of the catalytic elements is much smaller than the concentration which would normally be needed to effect crystallization in a thermal equilibrium state. Typically, the former concentration is less than one tenth the latter concentration.
More specifically, the crystallization can be promoted by setting the concentration of these catalytic elements to 1xc3x971015 to 1xc3x971019 cm xe2x88x923, preferably 1xc3x971016 to 5xc3x971017 cm3. The film is then irradiated with laser light of an appropriate energy or other equivalent intense light. The energy density of the laser light or other equivalent intense light varies, depending on the wavelength of the irradiating light, the pulse duration, the temperature of the amorphous silicon film (or crystalline silicon), and other factors. For example, if the temperature of the amorphous silicon is set to 100 to 450xc2x0 C., preferably 250 to 350xc2x0 C., then crystallization can be accomplished with a less concentration of catalytic elements.
In the present invention, an amorphous silicon film is formed by utilizing the features of crystallization using the above-described catalytic materials. A film made of a material containing the catalytic elements is in intimate contact with the amorphous silicon film, or the elements are added to the amorphous silicon film. Then, the amorphous film is irradiated with laser light or other equivalent intense light to crystallize the amorphous silicon film. At this time, the material containing the catalytic elements is brought into intimate contact with selected portions of a substrate or introduced into these portions. Subsequently, the film is irradiated or scanned with laser light or other equivalent intense light. In this way, silicon films having different degrees of crystallinity can be formed on the same substrate. It is also possible to make a preliminary anneal at 350-650xc2x0 C., preferably 400-550xc2x0 C., for 1 to 24 hours, preferably 2 to 8 hours, before the laser irradiation.
In this manner, the crystallinity can be improved. Furthermore, barriers at grain boundaries which could not be removed if only thermal annealing is conducted can be lowered. In addition, even amorphous components remaining at the grain boundaries can be crystallized. Where this method is adopted, even if the crystallinity achieved by thermal annealing is low, complete crystal can be accomplished by subsequent laser irradiation. Hence, the concentration of the used catalytic elements can be lowered.
In the present invention, the crystallinity of regions doped with catalytic elements is improved by the subsequent laser irradiation over the crystallinity of regions less doped with the catalytic elements, whether the preliminary anneal is carried out or not before the laser irradiation. Furthermore, the obtained TFTs show characteristics comparable or superior to those of TFTs fabricated by the conventional laser annealing in which an amorphous silicon film is irradiated with laser light. Additionally, these characteristics are obtained stably by making the energy of the laser light or other equivalent intense light less than the energy of laser light used in conventional laser annealing. On the other hand, regions not doped with the catalytic elements can be crystallized by laser irradiation. Also in this case, stable characteristics are derived by making the energy of the laser light or other equivalent intense light less than the energy of laser light used in conventional laser annealing. Of course, the characteristics of the region not doped with the catalytic elements are inferior to those of the regions doped with the catalytic elements.
By making use of these features, the regions lightly doped with the catalytic elements are used to form low-leakage current TFTs in the pixel circuits of an active-matrix circuit. The regions heavily doped with the catalytic elements can be used to form high-speed TFTs which are used in the peripheral driver circuit. As a result, a circuit comprising transistors that achieve both low-leakage current and high-speed operation which would normally be conflicting characteristics can be formed on a substrate.
In the present invention, it is necessary that the concentration of the catalytic elements in portions forming TFTs which are required to have low-leakage current be lower than the concentration of the catalytic elements in portions forming high-speed TFTs. In order to make their difference greater or to lower the leakage-current further, the concentration of the catalytic elements in the active regions of the TFTs which are required to have low-leakage current is preferably less than 1xc3x971015 cmxe2x88x923.
Other objects and features of the invention will appear in the course of the description thereof, which follows.